feat(recibos): split a receipt, sin fondos per receipt, keep a batch open

Three asks from the 2026-10-08 meeting, all on the OCR receipt queue.

- Split: one receipt paid by several customers (411 + 523 sharing water
  3014057) posts one charge per part. statement_document_splits holds free
  amounts that must sum to the receipt in cents, checked on save and again
  at confirm; "partes iguales" fills them in. Each part's row carries
  captureRef <docId>#<n>, so the double-post guard still holds.
- Sin fondos per receipt: StatementDocument.outstanding, toggled in review.
  At confirm a line is outstanding if its receipt or the whole check is.
- Open batch: POST /statements/batches/:id/files appends scans, numbered
  after the existing pages; a COMPLETED batch reopens. The confirm card
  prefills the check the batch was last posted against.

Verified on dev against a real batch: 1,892.00 split 946/946 across two
customers, a sibling receipt posted outstanding, second confirm refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/statements/statement.dto.ts

@@ -1,4 +1,6 @@
 import {
+  ArrayMaxSize,
+  IsArray,
   IsBoolean,
   IsEnum,
   IsInt,
@@ -6,7 +8,9 @@ import {
   IsOptional,
   IsString,
   MinLength,
+  ValidateNested,
 } from "class-validator";
+import { Type } from "class-transformer";
 import { Currency, ServiceKind, StatementDocumentStatus } from "@jorgecuadros/database";
 
 export class CreateStatementBatchDto {
@@ -27,6 +31,22 @@ export class ReviewDocumentDto {
   @IsOptional()
   @IsEnum(StatementDocumentStatus)
   status?: Extract<StatementDocumentStatus, "MATCHED" | "NEEDS_REVIEW" | "CONFIRMED">;
+  /** Sin fondos for this receipt alone. */
+  @IsOptional() @IsBoolean() outstanding?: boolean;
+}
+
+export class SplitPartDto {
+  @IsString() @MinLength(1) customerId!: string;
+  @IsNumber() amount!: number;
+}
+
+/** A receipt's split; an empty list removes it. */
+export class SetSplitsDto {
+  @IsArray()
+  @ArrayMaxSize(20)
+  @ValidateNested({ each: true })
+  @Type(() => SplitPartDto)
+  parts!: SplitPartDto[];
 }
 
 /**